European patent application EP-A 1 137 024 discloses an apparatus for monitoring and transmitting a switch position, for example for a high-voltage or medium-voltage switching system. FIG. 1 diagrammatically shows such an apparatus having a switch 1 which is connected to a coder 3 via an auxiliary voltage source 2. The coder 3 is connected to a decoder 5 via a transmitting means 4. The apparatus has the task of monitoring the position of the switch 1 and transmitting a level U0 of the auxiliary voltage, the coder 3 and the decoder 5 being DC-isolated by the transmitting means 4 comprising optical components, for example.
In this case, when the switch 1 is closed, the coder 3 is supplied with electrical energy solely by the auxiliary voltage source 2. The coder 3 generates a pulse-duration-modulated pulse sequence having a particular connection duration or pulse length, the connection duration being a function of the auxiliary voltage U0. For any desired small value of the auxiliary voltage U0, the connection duration is 50%, that is to say a pulse length corresponds to exactly half the cycle time. For increasingly large values of the auxiliary voltage U0, the connection duration decreases but does not undershoot a predefined minimum connection duration. The presence of the pulse-duration-modulated signal thus signifies that the switch is closed, and the connection duration indicates the value of the auxiliary voltage U0. The pulse-duration-modulated pulse sequence can be generated in a module specifically configured for coding (Application Specific Integrated Circuit ASIC). A module which is specifically configured for decoding and is independent of a downstream microprocessor (Field Programmable Gate Array FPGA) may likewise be provided for detecting and evaluating the signals.
The transmitting means for the pulse-duration-modulated signal suitably comprise an optocoupler which transfers an input current Iopto coming from the coder to an output voltage Uopto to be processed by the decoder. In the previously known pulse duration modulation method, it should be taken into account that a connection edge and a disconnection edge of a pulse are delayed differently by the optocoupler, in particular on the basis of the temperature, which may result in inaccuracies when determining the relevant times and thus the pulse length. For this reason, a delay of, for example, a rising edge of the output voltage of the optocoupler may be measured when calibrating the monitoring apparatus and a corresponding correction value may be introduced for operation.